Where can I find remaining time for preview render completion in 2.8?
Change the way the render will be displayed in Render (Blender's header) > Display Mode > Image Editor. 



Render again. Now see if a status bar appears inside Image Editor with the information you need.









To preview in Render Preview (Rendered Viewport Shading), enable Overlays and check the Text Info checkbox. Now the remaining samples will probably appear.